1. Field of the Invention
The invention concerns an illumination system for wavelengths ≦193 nanometers (nm), and more particularly, an illumination system for vacuum ultraviolet (VUV)-lithography or extreme ultraviolet (EUV)-lithography.
2. Description of the Related Art
In order to allow even further reduction in structural width of electronic components, especially to the submicron range, it is necessary to reduce the wavelength of the light used in microlithography.
For wavelengths smaller than 193 nm, lithography with weak x-rays or so-called EUV-lithography is discussed.
A suitable illumination system for EUV-lithography should homogeneously or uniformly illuminate, with as few reflections as possible, a pregiven field, especially an annular field of an objective lens, under lithography requirements. Furthermore a pupil of the objective lens should be illuminated up to a particular degree of filling σ, independently of the field, and an exit pupil of the illumination system should be situated in an entrance pupil of the objective lens.
Regarding the basic layout of EUV-illumination systems, we refer to European Patent Application No. EP 99 106348.8, submitted on Mar. 2, 1999, entitled “Illumination system, especially for EUV-lithography”, U.S. patent application Ser. No. 09/305,017, submitted on May 4, 1999 entitled “Illumination system particularly for EUV-lithography”, and International Patent Application No. PCT/EP 99/02999, submitted on May 4, 1999, entitled “Illumination system, especially for EUV-lithography”, the disclosure contents of which are incorporated in their entirety in the present application.
The following are discussed herein as light sources for EUV-illumination systems:    Laser plasma sources    pinch plasma sources    capillary discharge sources    synchrotron radiation sources
In the case of Laser plasma sources, an intensive Laser beam is focused onto a target (solid, gas jet, droplet). The target is heated so strongly by the excitation that a plasma is formed. This emits EUV-radiation.
Typical Laser plasma sources have a spherical beam, i.e., a radiation angle of 4 π, as well as a diameter of 50 micrometers (μm) to 200 μm.
In pinch plasma sources, the plasma is produced by means of electrical excitation.
Pinch plasma sources can be described as volume radiators (D=1.00 millimeters (mm)), which emit in 4 π, whereby a beam characteristic is dictated by source geometry.
Capillary discharge sources are pinch plasma sources, where a discharge plasma is produced inside of a capillary; such sources emit only into a small angular cone. Such a source is disclosed, for example, in Marc A. Klosner and William T. Silfvast, “High-temperature lithium metal-vapor capillary discharge extreme-ultraviolet source at 13.5 nm” in Applied Optics, Vol. 39, No. 21, Jul. 20, 2000, pp. 3678- 3682 and Hynn-Joon Shin, Dong-Eon Kim, Tong-Nyong Jee, “Soft X-Ray amplification in a capillary discharge” in Physical Review E, Vol. 50, No. 2 (1994), pp. 1376-1382. The disclosure content of these articles is incorporated herein by reference to the full extent.
In the case of synchrotron radiation sources, one can distinguish three types of sources at present:                bending magnets        wigglers        undulators        
In bending magnet sources, the electrons are deflected by a bending magnet and emit photon radiation.
Wiggler sources comprise a so-called wiggler for deflection of an electron or an electron beam. This wiggler comprises a multiple number of alternating polarized pairs of magnets arranged in rows. If an electron passes through a wiggler, it is subjected to a periodic, vertical magnetic field and the electron oscillates in a horizontal plane. Wigglers are also characterized by the fact that no coherency effects occur. The synchrotron radiation produced by a wiggler is similar to a bending magnet and radiates in a horizontal solid angle. In contrast to the bending magnet, it has a flux that is intensified by the number of poles of the wiggler.
There is no clear dividing line between wiggler sources and undulator sources.
In an undulator source, electrons are subjected to a magnetic field of shorter period and smaller magnetic field of the deflection poles than in the case of a wiggler, so that interference effects occur in the synchrotron radiation. The synchrotron radiation has a discontinuous spectrum based on the interference effects and emits both horizontally as well as vertically in a small solid-angle element; i.e., the radiation is highly directional.
It is critical for an EUV-illumination system to provide a sufficiently high Lagrange optical invariant or etendue, also called geometrical flux. The Lagrange optical invariant of a system is defined as a product of an illuminated surface and an area of an aperture.
If a numerical aperture in a plane of a wafer is in the range NAwafer=0.1-0.25, then in the case of 4:1 systems, a numerical aperture of NAreticle=0.025-0.0625 is needed in a reticle plane. If the illumination system is supposed to illuminate this aperture homogeneously and independent from the field up to a filling degree of σ=0.6, for example, the EUV-source must have the following 2-dim Lagrange optical invariant or etendue: (LC).LCill.=σ2LCObj=0.467 mm2−2.916 mm2 
The Lagrange optical invariant LC, is generally defined as follows for the lithography system described herein:
LCill.=σ2x·y·π·NA2=σ2A·π·NA2, wherein A is the illuminated area. The area comprises 110 mm×6 mm, for example, in the reticle plane.
The Etendue of a Laser plasma source can be defined as the product of an illuminated surface of an imaginary unit sphere around a source and a square of a Numerical Aperture at which each field point of the imaginary unit source sees the spherical source.LC=A·π·NA2 ALPQ=2π[cos(θ1)−cos(θ2)]×(Rsphere)2, with Rsphere=1 mmNA≈rLPQ/Rsphere=0.100where θ1 is a minimum beam angle with respect to an optical axis and θ2 is a maximum beam angle with respect to the optical axisLCLPQ=2π[cos(θ1)−cos(θ2)]·π·r2LPQ 
With typical source parameters:    1. rLPQ=0.1 mm, θ1=0°, θ2=90° yields: LCLPQ=0.198 mm2. This corresponds to 42% of the required value of the Lagrange optical invariant LCill of, for example, 0.467 mm2.    2. rLPQ=0.025 mm, 1θ1=0°, θ2=90° yields: LCLPQ=0.0123 mm2. This corresponds to 2.6% of the required value of the Lagrange optical invariant of, for example, LCill=0.467 mm2.
The Lagrange optical invariant LCPinch of a pinch plasma source with a diameter of 1 mm, Ω=0.3 sr, for example, is:LCPinch=A·π·NA2=(π·1 mm2/4)·π·0.305322=0.229 mm2.
Thus, the pinch plasma source provides 49% of the required value of the Lagrange optical invariant of, for example, LCill=0.467 mm2.
The Lagrange optical invariant or Etendue for the undulator source can be estimated by a simplified model assuming a homogeneous two-dimensional radiator with diameter    Ø=1.0 mm and aperture NAUnd=0.001 with    LCUnd=A·π·NA2     AUnd=π·(Ø/2)2             =0.785 mm2             NAUnd=0.001    as    LCUnd=A·π·NA2=2.5E−6 mm2.
As can be seen from this rough calculation, the Etendue or Lagrange optical invariant of the undulator source is much too small in comparison to the required value of the Lagrange optical invariant.
To increase the Lagrange optical invariant, an illumination system comprising a synchrotron radiation source known from U.S. Pat. No. 5,512,759, comprises a condenser system with a plurality of collecting mirrors, which collect the radiation emitted by the synchrotron radiation source and form it to an annular light beam that corresponds to an annular field being illuminated. By this, the annular field is illuminated very uniformly. The synchrotron radiation source has a beam divergence>100 millirad (mrad) in a plane of radiation.
U.S. Pat. No. 5,439,781 shows an illumination system with a synchrotron radiation source in which the Lagrange optical invariant, is adjusted by means of a scattering plate in an entrance pupil of an objective lens, wherein the scattering plate can comprise a plurality of pyramidal structures. Also, in U.S. Pat. No. 5,439,781, the synchrotron radiation source has a beam divergence>100 mrad. The synchrotron radiation according to U.S. Pat. No. 5,439,781 is also focused, for example, by means of a collector mirror.
The disclosure contents of both of the above-mentioned documents                U.S. Pat. No. 5,512,759        U.S. Pat. No. 5,439,781are incorporated into the disclosure contents of the present application by reference.        